1. Field of the Invention
The present invention relates to a pulse-tube refrigerator and more specifically, to a U-shaped stirling pulse-tube refrigerator.
2. Description of the Related Art
An in-line pulse-tube refrigerator in which a compressor, a regenerator and a pulse-tube are serially placed, or a U-shaped pulse-tube refrigerator in which a regenerator and a pulse-tube are juxtaposed with each other is known as a so-called stirling pulse-tube refrigerator (Japanese Laid-open Patent Publication No. 2001-289523, for example).
In such a stirling pulse-tube refrigerator, working frequency of working gas is on the order of a few dozen kHz and the working gas reciprocates in the refrigerator at an extremely high-speed. This feature is largely different from those of a so-called Gifford-McMahon pulse-tube refrigerator whose working frequency of working gas is about 1 to 2 Hz.
However, even for such a stirling pulse-tube refrigerator, it is still highly required to further improve the cooling efficiency.